Borehole tools can acquire subsurface data to aid locating and mapping of boundaries (e.g., bed boundaries) between layers of material, such as rock beds, and, for example, to visualize and orient fractures and faults.
A borehole tool may be configured to acquire electrical borehole images. As an example, the fullbore Formation MicroImager (FMI) tool (Schlumberger Limited, Houston, Tex.) can acquire borehole image data. A data acquisition sequence for such a tool can include running the tool into a borehole with acquisition pads closed, opening and pressing the pads against a wall of the borehole, delivering electrical current into the material defining the borehole while translating the tool in the borehole, and sensing current remotely, which is altered by interactions with the material.
Raw data can include multiple electrode readings, caliper readings from individual pads or pairs of pads, and x-, y-, and z-axis accelerometer and magnetometer readings. Borehole deviation and a first pad (e.g., pad 1 for the tool) orientation can be determined from magnetometers. A sample rate for electrode and accelerometer data can be on the order of about 120 samples/ft (400 samples/m).
Areal coverage of a borehole face can be a function of width of electrode arrays, number of pads, borehole diameter, etc. As an example, about 40 percent to about 80 percent of a borehole face may be imaged. Where data is not collected, so-called “non-imaged parts”, raw data may be separated by blank “strips” (e.g., between adjacent pads on a resulting borehole log).
Processing of current data sensed remotely in response to delivery of current in a borehole can provide a map of resistivity of a rock-fluid system at the borehole face (e.g., cylindrical borehole surface). For viewing borehole data, a line may be defined along a “true north” direction along which the “cylindrical” data is “split” between top and bottom and unrolled to provide a 2-D view. The line along which the “cylinder” is “split” may be any other geographical direction or may be the “Top of hole” or other such orientation.
For a boundary, if planar and at a non-orthogonal angle to the axis of the cylinder, the intersection between the boundary and a cylindrical borehole is an ellipse (e.g., in an extreme case, a plane that is exactly perpendicular to the axis of a cylinder would give an exact circle). Upon unrolling the cylindrical image of the borehole surface image, this oval is “cut” and open up as one cycle of a sinusoidal curve. Because the sinusoidal curve is part of an oriented image, every part of it corresponds to an orientation, and the lowermost part of the curve indicates the apparent dip (slope) azimuth (direction). The amplitude of the sinusoidal curve corresponds to a dip angle relative to the borehole, for example, where the greater the amplitude, the greater the dip angle relative to the borehole. On the other hand, in an extreme case, where the amplitude becomes zero, (i.e., a plane that is exactly perpendicular to the axis of a cylinder), the plane would appear as a straight line in an unrolled 2-D view.
Processing can include creating a series of borehole images where color maps are applied to different bins or ranges of resistivity values (e.g., for a tool that provides resistivity values). In the borehole image, color pixels can be arranged in their proper geometric position representing a borehole surface. One convention provides that low-resistivity features, such as shales or conductive minerals or conductive fluid-filled fractures or pore spaces, are displayed as dark colors; whereas, high-resistivity features, such as hydrocarbon-filled or well-cemented sandstones and limestones, are displayed as shades of yellow, and white—the higher the resistivity the brighter the image. As to a gray scale convention, black may correspond to low resistivity and white to high resistivity.
Processed borehole images may be of a static type or a dynamic type. Static images are those which have had one contrast setting applied to the entire borehole, which can provide useful views of relative changes in material resistivity. Dynamic images, which have had variable contrast applied in a moving window, can provide enhanced views of features such as vugs, fractures, and bed boundaries. Dynamic images tend to be better at bringing out subtle features in rocks that have very low resistivities, such as shales, and very high resistivities, such as carbonates and crystalline rocks or in any rocks with low relative contrast between the beds and other features.
Described herein are various examples of technologies and techniques that may transform borehole data.